In recent years, demand for engine-motor hybrid electric vehicles has been rapidly growing because of social needs for the improvement of fuel economy and reduction in exhaust emission. Many of the hybrid electric vehicles currently on the market are constructed as described in JP 2005-90307A. These vehicles include an internal combustion engine, a first motor-generator (MG) mainly used as a generator, and a second motor-generator (MG) that mainly drives wheels. In these vehicles, the crankshaft of the internal combustion engine is coupled with the carrier of the planetary gear of a planetary gear set as a power split device, the first MG is coupled with the sun gear of the planetary gear set, and the second MG and the drive shaft of wheels are coupled with its ring gear.
When the engine is cranked by the first MG to start the engine, part of the torque Tmg1 of the first MG (i.e., cranking torque) is transmitted to the drive shaft by the planetary gear set as illustrated in FIG. 2 and FIG. 3. At the same time, inertia torque (Kinr×Dne) arising from the inertia of the first MG and the engine is produced by change in rotation speed due to cranking. Therefore, cranking reaction force torque Tep equivalent to the sum of the torque (−Kgear×Tmg1) transmitted from the first MG and the inertia torque (Kinr×Dne) is exerted on the drive shaft.
As illustrated in FIG. 3 and FIG. 4, when the variation Dne (amount of increase in rotation speed) in the rotation speed of the engine fluctuates by variation in the cylinder pressure of the engine during cranking, the inertia torque fluctuates. In FIG. 4, EXP and COMP represent an explosion stroke and a compression stroke in each cylinders (#1, #2, #3, etc.) As a result, the cranking reaction force torque Tep exerted on the drive shaft fluctuates. Therefore, if no measure is taken, the torque Td of the drive shaft fluctuates and uncomfortable vehicle vibration may be adversely produced.
To cope with this, in JP 2005-90307A, when the engine is cranked by the first MG to start the engine, the torque of the second MG is so controlled that torque transmitted from the first MG to the drive shaft and inertia torque (pulsating torque) that fluctuates in conjunction with cranking are canceled out by the torque of the second MG. Thus, fluctuation in the torque of the drive shaft is thereby suppressed.
However, it is required to cancel out the inertia torque (pulsating torque) that largely fluctuates in conjunction with cranking by the torque of the second MG. In order to accurately cancel out the largely fluctuating inertia torque through control of the torque of second MG and thereby accurately suppress fluctuation in the torque of the drive shaft, it is required to shorten the operation period for control of the torque of the second MG. This leads to an increased computation load on a control device (electronic controller).